PRESIDENTIAL ADPRESS, 419 



Vorder and Hinter Hhein and the Lake of Constance, to the water flowing 

 through the trough-valley was probably an accident that occurred later than 

 the Riss-age of the Glacial epoch. The system indicated above, representing 

 the flow from a hundred miles of snow-clad mountains, must, however, have 

 made a remarkable change in the stream across the Armorican hills. In time, 

 as it lowered the loose deposits of the Mainz basin, this river carved out the 

 young ravine that runs like a knife-cut through the range ; the water that 

 flows past Mainz, with the exception, perhaps, of that from Constance, repre- 

 sents the magnitude of the event that we speak of as the diversion of a stream. 

 The abrupt change was not confined to the hill-region. When the Alpine water 

 arrived at the Mainz basin, and found its way into the notch formed by the 

 Pliocene Rhine, carrying with it mud from the glaciers of the Jungtrau and 

 coarser alluvium from the old trough-valley, it poured down upon the forest- 

 covered delta-land. 



The changes that have occurred in the unconsolidated ground of Holland in 

 historic times, including the loss of the Biesbosch, with its seventy-two villages, 

 in a single night, furnish some picture of what must have happened in the pre- 

 hi.storic delta of the Rhine. Land was suddenly built up at some points, island? 

 were carved out at others, and the effects of the catastrophe must have been 

 still manifest when the Scandinavian ice-sheet began to invade the mud-flats 

 from the north. 



The capture of a large river may be illustrated by the story of the Vistula. 

 This noble stream, the Rhine of Polish lands, represents in a remarkable way 

 the drainage of 190 miles of the Carpathians. All this water becomes con- 

 centrated, at the apex of a reversed river-fan, at the east end of the Kielce hills, 

 and it is probable that the upper Vistula was driven to join the San by the 

 advancing ice-front of the Riss-age, and that both rivers then escaped south- 

 wards. The joint waters were again held up when the Fennoscandian ice rested 

 along the line marked by the Baltic Heights, and it is well known that a great 

 river flowed westward along the stagnating ice-front where now the marshes 

 of the Netze mark its course. This river stretched away west to join the 

 Elbe, and the water from the Galician highlands thus met that from Switzer- 

 land in the Anglo-Danish delta-land. As the ice-front shrank backward, towards 

 the Baltic basin,*' streams flowed down over the sands and boulder-clays and 

 cut their valley-heads back southward. Overflows may have taken place on the 

 unconsolidated wall of the great east-and-west river, which was now deprived of 

 its barrier of moraine-filled ice. In one way or another, the shallow valley of 

 the main river was tapped near Kustrin, and the Oder, rising in the Moravian 

 plateau, was sent northward as an independent stream. Similarly, the Vistula 

 was carried off at Fordon, where the bend due to capture is conspicuous at the 

 present day ; and the whole drainage of the north wall of the Carpathians swept 

 across the drift deposits down the course of some hitherto unimportant stream. 

 Along the valley thus carved out, brown and yellow cliffs now rise above the 

 marshy flood-plain, and the red castles of advancing Germany have for centuries 

 looked down firmly on the stream. It is quite contrary to our customary 

 philosophy, but a good corrective all the same, to ask ourselves if this lower 

 valley of the Vistula, eighty miles in length, was shaped in a few months or a 

 few years. The main part of the excavation, across unconsolidated lands, may 

 have occupied less time than the building of the strongholds at the fords. 



Conclusion. 



In spite of the swamping of the Alkmaar coiuitry in 1825, in spite of the 

 tragedy of Messina only seven short years ago, we feel that Europe is a settled 

 continent, and we judge the past and future by the present superficial peace. 

 We have applied the same thoughts to human movements, and the inconceivable 

 has happened in our midst. We naturally find it difficult to carry our minds 

 back to epochs when the earth-blocks may have parted asunder as ice part.s 

 across the polar seas. We have, however, still very much to learn about causes 

 now in action ; and the mystery of the earth, and of our connexion with it, 



"■'' R. Lepsius (Geologie von Deufschland, Pt. 2, p. 511) urges that the sinking 

 of the floor of northern Europe led to this northward trend of the streams. 



■ £ 2 



